The present invention relates to a spark plug for use in internal combustion engines and, more particularly, to a spark plug having an electrode which is provided with a spark discharge portion made of a base metal resistant to wear caused by sparking.
In a conventional spark plug for use in internal combustion engines, as disclosed in Japanese Patent Unexamined Publication No. 57-180886, a spark discharge layer (referred to as "discharge layer" hereinafter) which is formed from a noble metal on the central and/or grounding electrode by, for example, resistance welding.
In operation, sparking discharge takes place on the discharge layer. The discharge layer made of a noble metal, however, exhibits a reduced amount of wear so as to enable the spark plug to be used for a long time.
The use of a noble metal as the material of the discharge layer, however, incurs a rise in the cost.
On the other hand, Japanese Patent Unexamined Publication No. 61-26748 discloses a spark plug having a discharge layer which is formed of an inexpensive tungsten alloy.
More specifically, in this spark plug, the discharge layer is formed of an alloy having a composition containing 15 to 40 wt % of chromium (Cr) and the balance substantially tungsten (W), or an alloy composed of 1 to 10 wt % of an element selected from a group which consists of silicon (S), aluminum (Al), nickel (Ni) and iron (Fe), and the balance substantially W.
The discharge layer formed on an electrode of a spark plug from the above-mentioned tungsten alloy, however, encounters a problem in that the tungsten alloy is oxidized at high temperature at an impractically high speed, with the result that the discharge layer is worn rapidly. Thus, the discharge layer formed from the tungsten alloy impairs the service life of the spark plug, although it contributes to a reduction in the cost.